The present invention relates to a vacuum gripper and, in particular, to a vacuum gripper for gently gripping work-pieces having sensitive surfaces, such as solar cells, wafers or panels for flat screens, and for gripping heavy glass plates or plates made of a material having a very smooth surface, which are stacked on top of each other and are to be removed from above.
Vacuum grippers are generally known and are constructively adapted to each case of application, that is, to the work-piece to be gripped. If work pieces having sensitive surfaces are to be gripped, special measures must be taken to prevent that surfaces from being damaged.
On principle, there are two kinds of vacuum grippers:
On the one hand, there are vacuum grippers which are put onto the work-piece and, at the beginning of the gripping procedure, are mechanically brought into contact with the surface of work-piece, that is, the gripper is provided with a sealing the lips of which effect a sealing action between the surface of work-piece and the gripper so that, when a vacuum is generated within the gripper, the barometric pressure causes the work-piece to be pressed against the sealing lips. However, due to the high contact pressure, the surface of work-piece can be changed undesirably.
On the other hand, there are vacuum grippers which are constructed such that they suck the work-piece to the gripper head from a certain distance and hold it in a non-contact way close to the gripping surface of the gripper. This effect is gained when the surface of the gripper is formed as an air bearing, for example. An air bearing can be generated by a field of nozzles from which compressed air flows out or by an oscillating plate such as described in the document EP 1387808 B1 which is completely included in the following description.
When the work-piece is sucked, it is urged to the gripper surface by the atmospheric pressure and keeps hanging at it. However, contacting of the gripper surface by the work-piece is prevented by the air bearing. That is, the work-piece is kept levitating under the gripper surface in a predetermined distance without contacting it. However, this functional description is in force only for the stationary state. Therefore, during the procedure of gripping, there is a problem which will be explained below:
In order to lift a work-piece by sucking, an increased suction power is required. However, it would be desirable to reduce the suction power before the work-piece reaches its final position, because otherwise, there is the danger that, caused by its mass force of inertia, the accelerated work-piece overcomes the force generated by the air bearing and hits against the surface of gripper.
In practice, it is often necessary to chose a maximum suction force 10 times greater than the holding force actually required, which, in turn, must slightly be greater than the weight of work-piece, that is, the mass thereof.
Also, there are gripping actions where the maximum suction force must still be greater. This is the case when glass plates have to be lifted separately from a stack or a glass plate has to be lifted from a table having a very smooth surface, for example. In such a case, almost no air is present between the glass plate and the surface of table. When the glass plate is lifted up, first if all, during lifting, the surrounding barometric pressure must be overcome to form a small gap into which air can enter. When the gap is completely filled with air und, thus, the pressure in the gap corresponds to the barometric pressure, the contact pressure initially generated becomes zero so that merely the weight of work-piece has to be overcome. The additional force necessary for separating smooth plates is called breakaway force in the following.
Therefore, a vacuum suction gripper sucking work-pieces without contacting them mechanically, has to be designed so that the suction force of the gripper overcomes the breakaway force and then, accelerates the work-piece towards the surface of gripper, against the gravitational force. It would be desirable to decelerate the work-piece and thus, to reduce its mass force of inertia as soon as it reaches a predetermined distance to the surface of gripper to prevent it from hitting against the gripper.
In an automatic process, as the work-pieces to be lifted have the same size and the same weight, it is theoretically possible to arrange a fast-reacting valve control means behind the suction nozzle, which, after the breakaway force has been overcome, reduces the suction flow in a first action so that the work-piece in the shape of a plate is decelerated to the technologically optimum speed. A second throttling action can be carried out after the plate has already been lifted a predetermined distance, in order to decelerate the plate such that it reaches its final position at the surface of gripper.
An expert knows that the control of such highly dynamic processes is extremely complex and thus costly, because it is difficult to control gaseous media, conditioned by the compressibility thereof.